Voice-frequency signaling system



Dec. 4, 1951 w. w. FRITSCHI ETAL VOICE-FREQUENCY SIGNALING SYSTEM 7 Sheets-Sheet 1 Filed Oct. 29, 1948 .W W FR/TSCH/ INVEN7ORS- C W LUCEK ATTORNEY Dec. 4, 1951 w. w. FRITSCHI ETAL 2,577,614

VOICE-FREQUENCY SIGNALING SYSTEM I Filed Oct. 29, 1948 '7 Sheets-Sheet 2 A 7' TORNE Y Dec. 4, 1951 w. w. FRlTSCHl ETAL [2,577,614

VOICE-FREQUENCY SIGNALING SYSTEM Filed Oct. 29, 1948 7 Sheets-Sheet 5 A T TORNE V Dec. 4, 1951' Filed Oct. 29, 1948 '7 Sheets-Sheet 4 J4. WFRITSCH/ INVENTORS. C. m LUCEK ATTORNEY Dec- 1951 w. w. FRITSCHI ETAL 2,577,614

VOICE-FREQUENCY SIGNALING SYSTEM Filed Oct. 29,1948 '7 Sheets-Sheet 5 3 l I b 1 $1 I I ,I

m t I m m w w W WFR/TSCH/ lNVENTORS. a WLUCEA,

BY @am ATTORNEY Dec. 4, 1951 w. w. FRITSCHI ETAL VOICE-FREQUENCY SIGNALING SYSTEM 7 Sheets-Sheet 6 Filed Oct. 29, 1948 W FR/TSCH/ wvewrogi v C W LUCEK ATTORNEY Dec. 4, 1951 W. W. FRITSCHI ET AL VOICE-FREQUENCY SIGNALING SYSTEM 7 Shets-Sheet '7 I WWFR/TSCH/ N .3? ESQSQQ Filed Oct. 29, 1948 who A TTO/PNEV Patented Dec. 4, 1951 7 2,577,614 7 VOICE-FREQUENCY SIGN ALING' SYSTEM Walter Wi'Fritschi, Manhasset, and Oharles Lucek, Port Chester, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation oit Newyork" Application October 29, 1948, Serial No. 57,384

, I 1 .Thisfinvention relates to signaling systems and particularly to telephone systems wherein voice frequency currents are employed in the 'transmission of selective and supervisory signals.

,Objects of the invention are the provision of a more reliable and more'stablesignal transmitting andreceiving means in systems employing voice frequency signaling, the provision of such signaling means for the transmission of signal's over both two-wire and four-wire lines, the prevention '6 Claims. worm- 1 outward transmission 0 of {false operation in response'to voice currents or other interfering currents, and the provision of means to insure tone signals of minimum and maximum duration to insure proper signaling.

The invention comprises a voice frequency signalin "'s'y'stem in which sigialsare transmitted by the initiationand by the termination of transmission of alternating current and by impulses of alternating current, and in which theloperation of signal responsive devices is delayed to prevent ialseopera'tion dueto voice currents or other hannels associated therewith, ;all aiding in improving the reliability of proper'operation of the signaling circuits under I widely differing signaling conditions and aiding in preventing the operation of said circuits byv signal .frequencyrin'voice currents and aiding in minimizing-.the elfect upon said circuits of audible tonesinot comprising signalin frequency.

Complete understanding of the invention will szbe realized from the subsequent description of a typicaltelephone system embodying the invention; Such asystem is disclosed by example in thedra'wings 'forming a part of this specification and describable generally as follows:

interfer currents and in which signals otherw of risu'fiicient duration to effect'theoperatio said devices are lengthened to insuresuch operation,

tov H frequency signals for controlling'signaleviceai' said, circuit provided with delay that such signalsare of sufiicient duration ot -ien'otheir feature of the present invention is the provision of a voice frequency transmitting cir'-' cuitarranged, in response to externally created signa ling conditions, to transmit voice frequency signals of various degrees of duration to which electronic receiving circuits are intended to be a responsive, and means for augmenting excessively short durations of signaling conditions by forcing said transmitting circuit to transmit minimum durations of voice frequency signals irrespective of the durations 'of said signaling conditions controlling said transmittingcircuits to insure response thereto by said receivin c rcults.

. Furth may-control the sensitivity and frequency selectivityv of the associated receiving circuits, and;

means; whereby the signaling circuits may control-the insertion and removal of blocking'or band el mination filters i to-.01- out ofthe inward and one nature of the", invention is the provision ofan electronic signal receiving circuitresponsive s'to prevent response thereby to short dura O 'f. signal frequency in voice currentsin the herwiseito effect response thereto by said cir-' features of the invention areprovisions of means whereby the signaling circuits Figs. 1', 2 and 3 show a first toll ofiice wherein; Fig. l shows an operators position OPI, part of an outgoing'trunk circuit O-Tl, a two-way trunk circuit TWTI', a selector TSI, and an incoming trunk circuit I'Il Fig. 2-shows part of the outgoing trunk circuit 'OTI, avoice frequency signal generatorfillfl', part of the incoming transmission channel from the toll line, and various control relays associated with the signal transmitting andreceiving circuits; Fig. 3 shows the electronic signal receiving'circuit and part of the incoming transmission channel from the toll line;'

Fi s. '4, 5 and 6 show a second toll ofilce wherein Fig.6 shows anoperators position 0P2, part of I anoutgoin'g trunk 'circuitOTZ, a two-way trunk 'circuitiTWTZ, a selector TS2, and an incoming trunk circuit IT2; Fig. 4 shows part of the outgoing trunk-circuit 0T2, a voice frequency signal generator 400, part of the incoming transmission channel from the toll line, and various control relays associated with the signal trans,-

mitting and receiving circuits; Fig. 5 shows the electronic receiving circuit and part of the incoming transmission channel from the toll line;

rangement;

Fig: 4B shows a typical two-wire toll line rangement; and

Fig. '7 illustrates how Figs. 1 to 6 and 4A or 4B may be arranged to form an operative system.

GENERAL SYSTEM DESCRIPTION Thesy stein represented in the drawings in- .cludes a pluralityof toll ofices each of which comprise artoll board with operators positions and cords for answering calls incoming from manual or diallocal ofiices, from toll subscribers 1 lines, fromcommunity; dial ofiices, or from other 55 1011 oflices; and for connecting intertoll trunks Fig. 4A- shows a typical four-wire tollline ar-';

ar- L ond selectors, if required, are providediorruse on;

calls incoming over intertoll trunlis' to" establish connections with other toll, local orcommunity dial ofiices. The operators po's' ions, are provided with dials for use in controlling thelop eration of intertoll selectors in other toll oflices and fo'r'controlling the operation of selector ,and; connector switches in local or community dial offices Reference may be made to Patents 2,209,777 to R. E. King and O. R. Miller of July 30, 1940,

and 25,06,236 to J E. -Wa1Sh' of December 22, 1942,

for alcon'ip lete disclosure of the cord and opera tors position circuits provided in each ofv the-two tollofiices for interconnecting calling and called lines .and trunksand for controlling the comple-i tionpofltoll' cal1s.- The outgoing trunk circuits OT.l wand 0T2, incoming, trunk circuit ITI and 1T2, twoi-iwayftrunk circuits TWTI and eTWTZ,

andthe .tollrouteselector circuits TS! and T82 V are all similar tox.corresponding circuits ofthe aforementioned King-Miller andWalsh-r patents. The two toll offices shown in the drawings are nterconnep e rb e he o. r.rw e l nesor i e l es o a ombina ion o ho h- Onetwo: wire l e is, sh wn in Pia Bland com ises e or t ll inef s ieted wit t u uts Q and W l i the me. n Qf d with trunk circuits 0T2 andJTWT'L' in the second i T. One:four wire lineis lalso shown in klnand em nse twe two w'ire lines. TLI and TL? associated w ith' 'the 'abovementi oned illustrations 'pfrig r 4n and "14B a e to e derstood as indicating" that four-wire. and T two-wire line'operationis 'alterniaitiye. In the former case, line TL] servesto trans'rrlit'voice and fvoice fref quency signaling current from the first toll oifice miflilse j n ithe l ie T Se es to a t Pik l v e 'f s u ln na i cu r t mm the second t'olloflicev tome first; In case'of twore ne per tien eiT v lt it e sr tre j e 151 i e e i l he n bo' ji i eF Pi nsj sig a n cPet ti ns b n performed at difierent frequencieswhich-are sejle'cted and blocked' as hereinafter described in order that the'two-wire line be -capable of operating'into the first andsecond toll oilices arranged as four-wire terminations for the twowire intertoll line without undesired infiltration of t he wrong signaling frequency into the wrong circuits at the wrong time. A hybrid coil HCl individual to trunk circuits O I-l and IWTl is arranged to transmit voice currentfrom these trunkcircuits over toll line TLI or-TL to thesecond tolliofiice and to receive...and.transmityoice current incoming over toll line TLZ or TL to these trunk circuits. A like hybrid coil HCZ- individual to trunk circuit-s QTZ and TWTZ is arranged to transmit voice current from the s'etru nk circuits over ten line TL: "or T tetn 'nrs'ttciucfricean t0 r eceive an'd transmit voice current incoming over toll line TLI 'or TL to these circuits. Balancing networks BNI and BN2 are connected;

toifliybrid coils HO! and HC-Zin the usual} and well-'k'nownmanner'.

0T1, OTLTWT! and Twrrzij The GENERAL DESCRIPTION OF SIGNAL TRANS- MITTERS The signal transmitter at the first toll office comprises a generator 200 of alternating current of a particular voice frequency, for instance, 1600 cycles .per second,.@ relay 20;! responsive to seizure, selectiye superyisory and disconnec t signals which are to be transmitted over the toll line TL! or TL, a signal tone level control relay 202 and a cut-off relay 203 for effectively disconnecting and terminating ,the toll line TLI in Fig. 4A or the equivalent one-way amplifier channel 'ILlf of Fig. 4B. associated with toll line TL, to efiectively n'regent noise originating in the local trunkpireuit from being transmitted over the toll line TLI or over toll line TL from channel TLI during thetransmission of signals outgoing over the toll line. As will be explained, a, low level 1600-cycle per second continuous tone signal is transmitted over the tollline TLI orTl; fr om the ts i' l f v he sewed t l si c imitat that the'first oiiice is receptive to accept calls. 'ii -w y am l fie cqml si ie va uum e. 9 n ,wmt iz i ie br cl1 9 HG tothe oll in TLI, and lone-wayiamplifier comprising vacuum 7 tub 3% andr n ect n t l B 1 7 1? hannel TL2, toihybr id coil H Cl preyent the transmission of i e H W i nt f om the Si nal t an mit er at the first toll office over toll line Til; or char nel LZ'. Amplifier ieflll'i ectivefu hgri o' r ven n ise v pn din hev loc ircu t irqm ect th ece ve by t eneh hrid .1 urrent I-hY QrQi HQ lur h x ore in th re' of a trgkw re t ll ne Tleuafi te c rcui 2. 09 i W tllWMl-TLZ" 9199 3. 35 Phi? transmission of S gnaling frequency i current of lfiOQflcycles per secondiimm channel TLI. to hannel hrou h hyb d. 9911 R 3.-

The signal transmitter at the second 'toll ,oifice is similar towthe one outlined briefly abovein connection with the first'toll ofiice corresponding elements, having referencecharacters with the same tensjandr'units digits. ThusisignaIing currentfromgenerator 400 is normally transmitted t, a 1ow.1eve1..ov r t ll line TM or channel 1L2 nd t li IL Th n lin i e rummy en eted b s i ratpr .ifi 'mar be 0 es. pe secon h. s me. as. ge erated by ee e et rl fly in thecase' of a tour-wire line such as TLl, Th2;

PW ven n e ev nt o a twmwire line s ch.

as. TL of Fig 4B; the si nalin equ n y en: r'atl b se ratpr 49 1 11 b ff ent say 2000' cycles per second," as will be apparent from" ubs quent descr ption I t e latt r case filte cu t 050 wil l ss, 2990103 h r e o u i t fiilfi c cle er c nd, unle s r a 29 n atesto. limina e the; bend limi a on ch eri i sof fil er. 09" a wi l b d scusse L ke wis fi ter rcu @5 9 i ass fiOO cles r ee ut 1 9? 299.0 cy s e ccn ll se se nder the cent e! i e a 3091- urthe mor ne-War amp f er gq pri ins acu m t e A a win ,hybrn Q 1H Zt the1t l 11 ea 91 QPWPWQ @9 1 .1. TL e t an TLI dro 99113.3 PZ'? the r nsmi of nalingcurrent from the signal transmitter at the second toll oilice over toll line TLI or channel" Tl lfl' Amplifier 500; as was the case with amplifier 360 specifically excludes noise generated in the localcircuits from afffecting the receiver;

Iii-addition, as above-indicated, in the case of a:

two-wire ton n TL, a filter circuit anon is-inserted intochannel TLH for blocking thetrans:

missionof signaling frequency current of 2000 channel" The signal receiver at the first toll ofiic'egshown mainly in Fig. 3, includes a vacuum tube-amplifier represented by vacuum tube 301 whose input, circuit includes one or both of the left-hand windings of transformer205 which terminates either toll line TL2 of Fig. 4A or channel TL2 of Fig. 4B. Sensitivity relay 206, when releasedf includes both of the left-hand windings of transi former 205-in the grid-cathode'input circuitof tube 301; and, when operated, includes'only the lower left-hand winding of transformer=205 in said input circuit, the upper left-hand winding being unused and terminated by resistance 20'! over the No. 5 armature andfront contact of-the sensitivity relay 206. The condition-of-operation of relay 206 and the adjustment'of .potentiometer 302 determine the amount of input signal frequency to amplifier 301 or the sensitivity of the receiver. The output from amplifier 301 is transmitted through a transformer 303 to a load comprising a volume limiting varistor arrangement 304, a low-pass filter comprising a retard coil 305 and condensers 306 and 301, and a frequency discriminating network 312 including a resonant section 315 of inductance 308 and condenser 309 and an anti-resonant section 316 of inductance 310and condenser 311,'each sec tion tuned or resonant to the signal frequency incoming over toll line TL2 or channel TL2.

\ Each of the sections 315'and 316 of network 312 is shunted by a rectifying arrangement including respectively condensers 314 and 313 which accu-r mulate charges corresponding respectively to'the. voltages appearing across the sections 315 and 316. These latter voltages are respectively representative of the amounts of signal frequency and other frequencies presented to them from the. output of tube 301. The charges on condensers- 313 and 314 are effective in controlling thegrid voltage of tube 311 such that if signal frequency. predominates, or exists alone in sufficient amount, thegrid voltage of tube 311 is sufficiently positive with respect to the cathode such that the plate current permitted to flow will operate the remove-filter relay 318 and the receiver relay 319. On the other hand, if other freouienciesv than the signal frequency predominate,-or exist alone, the grid voltage of tube 311 is insui'liciently positive to permitthe operation of relays 318 and 319. Both of the relay 318 and 319 are shown in their operated conditions as a result of low level signal frequency input to amplifier 301 representing a signal from the second tolloffice that it is in an idle condition with respect to toll line TL2 or channel TL2 and toll line TL. The operation of relay 319 completes the operatingcircuit for regeneration relay 208 which in turn.

removes ground from lead 110 at itsarmature in, circuit to incoming trunk circuit 1T1, of the first. toll office. Furthermore, relay 319, in operating, operates the cut-off relay 203 to terminate the repeat coil in the outward transmission channel to toll line TLI or channel TLI', to toll line TL as'hereinbefore outlined. I

When, therefore, low level signal frequencyis present in the incoming transmission channel from toll line TL2 or toll line TLover channel TL2 indicating that such channelis'idle with respect to the second toll office, the relays 318 and 319 of the signal receiver at the first toll oflice are operated,- as well as relays 208 and 203 controlled in part thereby.

In addition to the above outlined signal re ceiver comprising vacuum tubes 301 and 311 and:

relays 318 and 319, the inward transmissionchannel is provided with the previously mentioned amplifier tube 300 and associated circuit.

The input signal for said tube 300 is derived from the lower right-hand winding of transformer 205 and is confronted by a blocking net-- work-320 consisting of two sections 321 and 322 switchable into and out of the input circuit under the control of the remove-filter relay 318 and the.

aforementioned sensitivity relay 206. Section 321 of filter network 320 includes inductance 323 and condenser 324 in an anti-resonant circuit. Sec-- tion 322 of filter network 320 includes inductance 325 and condenser 326 in a resonant circuit. Both sections are resonant at the incoming signal frequency and, therefore, in effect amount to a band elimination filter most e-fiicient at the incoming signal frequency. The prime purpose of network 320 is, as will beapparent, to exclude signaling frequency signals from the hybrid coil HCI at the proper instants to exclude such sig- 312 in the input circuit for tube 311 the dischargecircuits for condensers 313 and 314 are arranged to provide a sluggishness in the operation of tube 311 for the purpose mainly of preventing receiver operation in response to very short spurts of pure signal frequency which may be incorporated in voice currents. These discharge circuits, by means of proper proportions of resistances 342 and 343, as will be explained later, also proportion the amounts of pure signal frequency and other frequencies affecting the tube 311.

Also, additional means for preventing longer spurts of pure signal tone from affecting the signal lead to the local trunk circuits is provided in the form of a mechanical delay in the cooperative operations and releases of relays 319, 206, 208 and 289. magnetic relay delay does not prevent the operation of the remove-filter relay 318 to cut the blocking network 320 into the inward transmission channel on the receipt of longer spurts of signal tone in the voice currents but does prevent false signaling of the local trunk circuits.

Since the above delays, particularly the slow operation of relay 319 and the actual receiver delays, Which may be considered electronic delays in that they effect foreshortening of any pulses orspurts of signal tone incoming over themforeshorten actual signal pulses as Well as spurts of pure signal tone in the voice currents. compensate for actual full-sized signal pulses which are foreshortened and for actual pulses of signal frequency current which might be of too short a duration to operate regeneration relay 208 for a long enough period of time to properly signal the local trunk oversignal lead 110, relay 208 is provided with winding circuit arrange-'-' 'gments whereby irrespective ofthe aforesaid fore- Such mechanical or electrocoming transmission channel, will, of course,'

gamers shortening of" signal pulses relay 12-08 will trans.- mit to the local trunk circuit a normal: duration: of direct: current signal a over: 'l'eadi l 110 Furthermore, incoming pulses of signal tone. whichmay be ofgreater: duration than required: for a .proper signal' to the local trunk circuit. will, by reason of the aforementioned electronic delays, be shortened.

The -signal receiver of: the: second toll office is shownmainl y in Fig. 5. and is the same as the:

signal receiver previouslyfoutlined in connection with: the first toll office. The signal receiver-at the second toll ofii'ce is provided. with reference numerals having the same tens. and unitsd-igits as corresponding elements of its counterpart in theefirst toll oflice.

DETAILED DESCRIPTION it is. believed that a fullunderstanding of the present invention will be realized most easily.

from a descriptionof the detailed o'perationot the disclosed system under various conditions encounterable during a typical toll call. The following description isprovided with that. purpose in view.

- I dZe condition of all circuits All circuits and relays therein. are shown in their idle conditions, i. e., when .all circuits are availableto initiate or receive calls, except for cut-off relay 253 which under idle conditions is operated in. a circuit from ground and battery, through the winding of relay 2B3, varistor 2!!! in the. low resistance direction, over the .No. 5 a1?- mature and back contact of relay 2.02, over conductors 2H and 252, over the upper normal contacts of the R test jack to ground over the front contacts of the-operated receiver relay 3|.9, the operation of which relay 3!?! will be explained hereinafter and has been outlined previously.

Polar relay I40 of outgoing trunk circuit OT! and polar relay it!!! of two-Way trunk. circuit TWT'I, being energizedonly by means of their lower windings in .obvious circuits, are conditioned as shown such that their armatures are in their left-handpositions. 7

Regeneration relay 2% is operated. as shown when the circuits areidl'e. The operating path may be traced from ground at the front contacts of the operated receiver relay 319, 'over the lower normal contacts of the R test jack, over conductor 3215 over the No. 1 armature and back contact of sensitivity relay 205, overconductor 2I3, through. resistance 2%, varistor H5 in the low resistance direction and to battery and ground through the lower winding or" relay 208. Relay 208, in operating, disconnects the relay MU, signal leadffi; through resistance 2H3,

conductor-2 1.1 to the. mid-point of resistance 2 E8. The :leftehandterminals. of each of the varistors ZJSPaI-id "2-10,, therefore, have. a resistance path towgroundat=the mid-point of resistance 221 aandltofground-iat the conductor 2 I 2' connected to the mid-pointof resistance -2i8. On theright- 7 hand side of -varistors M9- and 2 28 a negative signallead Hi) to the local trunk circuits fromv thatits armature is in its left-hand position as,

shown. The circuitsfor'relays 318 and 3l9are, asheretofore outlined and as will be explained further hereinafter, included in the plate-cathode circuit of tube 3.

The signal generator 258, under idle condi tion of the associated trunk circuit, isarranged to transmit over toll line TL! through repeat coil'2002 alow level of signal'frequency ofsay 1600rcycles per second. Signal lead 1.64 from the-local trunk circuit is grounded at the back.

potentialto-ground exists by virtue of the potentiometer arrangement comprising resistances 2,22- and: 2-23,.and. batteryJZd. A small current is permittedetqcfiow through varistors 2l9and 220 in.-the:forward,.-or left-to. right, direction which is; thetlow" resistance direction, thereby causing theresistance of---the'varistors to become relatively small" The; output of signal frequency, 1 01 say 1600 cyclesper second, is transmitted through condensers :225; and 2'26, varistors 2 i 9 and 2211, condensers 2 2! and-22 d and resistances 229 and 23ll-to -:respectiveconductors 231 and; 232;- and over {tip conductor- 23-3 and ring conductor 234, and

through repeatooil 280.2 to toll; line TLI to the second-. tolloffice. "Due to the presence in-the latter circuit of. "seriesresistances 2'29, and 2'30 the-level ofa'thesig-nalfrequency transmitted over the. toll facility TL is. relatively low.

,In .the casecf a two-wire toll. facility, as in FigL-AB', the low level :signal tone is. transmitted oVeracha-nnel TLI;v through one-way amplifier 'ZBQSEand throughahybiidcoil HC31and over 12011 line TLvtoi the second rtoll ofiice.

lie-previously mentioned, "filter circuit; I 2800 comprises. a. ibandjelimination filter or blocking network atlthezirequency offtthe'signal tone transrri'itted; from-signal generator fiiil' in the first toll ofiicep' Aswill-ibe obvious; operation of the filter relay infil -will: rem'ovelthe resonantv section 296 from its ibri'dglng connection across the inward transmission channel and will short circuit the two series parallel resonantsecti'ons to eliminate theililtering action of filter 2858'.

In -ei-ther caseof' two-wire or four-wire toll line operation the operated cut-'ofirelay 2&3 efiectivelycuts on the transmission channel comprisingtip-:conductor'2-33 and ring conductor 234 from the' local trunk circuit by bridging a terminating": resistance 23'5' thereacross over the upper No. 1 front contacts of relay 2533 and by inserting resistances 235 and 231 in series withthe tip-and ring conductors 233 and 234 from the secondary 'windi ng of transformer 238 and by inserting a bridging or terminating resistance 23s across the secondary Winding'of transformer 238 -.--Upon the release of cut-off relay 2B3, as will be obvious-each of the aforementioned seriesor brid'ging or terminating resistances is either short 'circuited over back contacts of relay-103 01 is isolatedinan open circuit to thereby leave a -dir'edttermination of transformer 238 by the ohannelcomprising conductors 233 and 234'fandirepeatcoirz'fltz orby the channel comprising conductors 233 and'2 3 l; channel TLI, amplifier :20n3rand t-hybrid coil HC3-.

The above described continuous low level sig nal frequency tone is transmitted from the first toll ioffice' -tothe second'toll office to signal the latter thatthe first tolloffice is idle and is'not in a calling condition and can be called. The idlecondi'tion at the second toll oflice is the same as -at the first toll. 'office and, therefore, signal generator can will transmit a low level signal tone through varistors M9 and 420, through series resistances 429 and 430, over conductors 43! and 432, over tip conductor 433 and'ring conductor 434, and through repeat coil 3002 to toll line TL2 to the first toll ofiice in the case of a four-wire toll facility, or over channel TL2', through amplifier 3003, and through hybrid .coil H04 and over toll line TL to the first toll oflice in the caseof a two-wire toll line.

The low level signal tone incoming to the first toll ofiice over toll line TL2 is transmitted through transformer 205 to the input of the signal receiver 'of the first toll ofiice represented by the input circuit of tube 30! and to the input circuit of the one-way amplifier represented by tube 300 in the incoming transmission channel to hybrid coil HCl and therefrom to the local trunk circuit. As will be understood, little of the signal tone is transmitted to coil HCl due to the blocking action of the filter 320. As previously outlined the. sensitivity or level of the input stage of the signal receiver is controlled by the condition of the sensitivity relay 206 and by the position of potentiometer 302. When relay 206 is released the grid circuit of tube 30I is energized by the entire secondary winding of transformer 205 in a circuit extending from the negative terminal of battery 323 through potentiometer 302 and resistance 329,'over lead 330 through the whole secondary winding of transformer 205, over lead V 240, over the No. 5 armature and back contact of relay 206, and over lead 24! back to thenegative terminal of battery 328. When sensitivity relay 206 is operated the same grid circuit of tube 30! is energized by only the bottom half of the secondary winding of transformer 205 in a circuit extending from the negative terminal of battery 328, through potentiometer 302 and resistance 329, over lead 330, through the lower half'of 'the secondary winding of transformer 205, over former 205 is shunted by resistance 20! when relay.

206*is operated in order to provide a load termination for the unused voltages appearing thereacross.

Tube 30| is self-biased by resistor 33l shunted by condenser 332 and is provided with a grid resistor 333 shunted bya condenser 334 to limit such that when high level pure signal tone is received the discriminatingnetwork 3l2 will not be unduly influenced thereby as will be appreciated from subsequent description.

Detailed description of receiver It is considered advisable at this point to describein detail the action of the receiver and its novel characteristics under certain input conditions so that a full understanding will have been acquired as to its functioning when the description of the idle and other conditions of the receiving circuits is continued hereinafter.

The discriminating network 3| 2 contains two sections. One anti-resonant or parallel resonant section 3| 6 between terminals I and 2 is composed of inductance 3l0 and capacitance 3H and is arranged to be resonant'in parallel at the frequency of signal current transmitted from signal generator 400 at the second toll office.v The other resonant or series resonant section 3l5 between terminals 3 and 4 is composed of inductance 308 and condenser 309 and is arranged to be resonant in series at the frequency of signal current transmitted from the secondtoll oflice. As will be remembered, such signal frequency may be the same, say 1600 cycles per second, as that transmitted from signal generator 200 at the first toll ofiice in case a four-wire toll line is used as shown in Fig. 4A, but must be different, say 2000 cycles per second, in case a two-wire toll facility is used as shown in. Fig. 43. Section 13l5 is normally shunted by resistance 243 in a circuit extending from terminal 3 of section 3l5, over conductor 335, through resistance 243, over the No. 4 armature and back contact of sensitivity relay 206 and over conductor 244 to terminal 4 T of section 3 I 5. The section 3| 6 will present a high impedance to pure signal frequency and a relatively low impedance to other frequencies. The

whenever the sensitivityrelay 206 is released,

limits the impedance of section 3l5 to substantially a maximum of the value of resistance 243; but,when the sensitivity relay 206 is operated, to thereby remove resistance 243 from its shunting relationship with section 3! 5, section 3l5 may reach values of impedance at frequencies differing grid current on high level voice and signaling currents. The incoming signal may be substantially zero or almost pure signal frequency or almost purely other frequencies than the signal frequency transmitted from the second toll office or a combination of both signal and other frequencies. This input signal is amplified by tube 30! and transmitted through transformer 303 to a load comprising volume limiting varistors:

304, a low-pass filter composed of retard coil 305 more and more from pure signal frequency that are many times the value of resistance 243. The removal of resistance 243 from its shunt path across section 3l5, therefore, permits a greater amount of voltage to develop across section 3l5 due to the presence of frequencies other than pure signal frequency' For convenience of terminology, the section 3|5 may be referred to as and condensers'306 and 301, and the discriminating network 3l2 with associated circuit elements.

The varistor arrangement 304, as indicated, isarranged to provide alow resistance shunt in?- both directions for signal voltages of a certain amplitude to thereby limit the peak-to-peak voltage of the signals transmitted to the low-pass filter andtothe discriminating network 3l2.

These signalsyofcourse, will be composed of high:-

frequency components and harmonics due to the tions of tube 30L the guard channel and the section 3|6, asthe signal channel and the reasons for such' terminology will be apparent from subsequent de- 'scription. Briefly, signal channel relates to section 3| 6 because it will produce its greatest voltage at the pure signal frequency of 1600 or 2000 cycles per second and guard channel re-- lates to section 3l5 because it will produce its greatest voltage at frequencies other than pure signal frequency, the latter statement depending, of course, on the shunting action of resistance 243 under control of the sensitivity. relay 206. "The effect of discriminating network 3l2' is further under the control of the guard removal relay 209;. The actual transmission path from the top plate of condenser 306 of the low-pass filter 1'1 extending to terminals. of1section3 IzIi-v is traced over conductor 3.3.6., over the unoperatedNo. 6 armature and b'ack-.contact. of vrelay 289', and over conductorr331=toterminal. I of section SIS.

When guard removal relay 209 is. operated the latter circuit is made toinclude resistance 245 since the No. -6 armature and back contact of relay 209' are no longer short-eircuiting resistance 245. Furthermore, relay 212th in operating, short-circuitssection M by connecting conductor 335:

to conductor 264 over the. No. '5 armature and frontcontact-of relay 209. The including of resistance 245 m series-with section 3|? and the short-circuiting of section '3I5 changes the characteristics of the discriminating network 3|? tov make it non-selective to incoming frequencies.

Each section 3th and 3I5 of the discriminating 'network'sil r-is shuntedi by a halt-wave rectifying arrangement; Section '3 l t is shunted by the rectifyingarrangementcomprising 'varistor 338 in parallel 'with resistance. 339 and in series with :cond'enseri3If3. Section'3 [5is shunted'by asimilar arrangement? of 'varistor't'34fl 'iniparal'lel with resistance 3M :and' inseries with condenser 3M.

Thexresistances 23 3-9.;and. 3544 are ns'ed'tinra-n efiort:

to equalize the lBSDBCfiVGZIBSiStBJIICGS 'lof varistors 338 and 3% inthe :reverse'ior high resistance direction. It 'willbe'iapparent that an alternating current'voltage' will appear 'across' each section 3I5 and '316 of '*the*discriminating network 3I2 and, asa result of the rectifyingaction of the above described rectifying'arrangements shunting sections s'I E'and 31%}2', direct current Voltage will appear"acrossscondenser 313 -and another direct current voltage'wiil appear across 0011-.

denser 3M, 'therespective direct current Voltage "representing signal. channel voltage and guard channel voltage." If the received signal containspnly the virequency ofthe signaling tone a relatively large voltage will appear across con- "denser 3 I3.- If, however, thereceived signal con-- tains only frequencies other than the signal tone frequency a'relative'ly large voltage Wi'li appear across condenser 3 I A. If bothitypes lof frequencies "are present there will be represented by 'corre- :spon'ding voltages on both condensers "-3I3 and 3M the-relative power :and frequencies of the variouscomponents in the composite signal.

The'varistors'333 and 3411; Whenpo'led as shown and when understood to pass currentat a low resistancein the :iorwarddirec-tion '(directionof arrowhead) when underthe influence-10f a voltage pressure attempting to? force current :in that direction, will effect a positive charge-on the top plateof. condenser- Sis and a negative charge on the bottom plate of condenser 3. The voltages on condensers 31 3 and 344 have "a combined effect, by means of'resistanceMZ andresistance 343, Withl-its :shunti-ngcir-cuit of resistance 344; varistor 345 and condenser :3 M3,, on the grid input voltagepftube 3H. Condensenflfleonnected to the upper plate of condenser313 has "an effect upon the combined contro'l ofthe gridivoltage'of tube 31 T'as will be-explain'ed' later.

The grid-cathode biason tube 3-1! is derived from the'algebraic sumflof the voltages in the path extending from the gridof tube -3-I I; through r2 ceiver :is pure signal tone frequencmaa voltage'mill be developed. across condenser 31 3, resultingin 1a discharge currentifrom the :top positive; plate of con-denser3i3, through resistance 342, through resistance: 353,- through .varistor .348, "over :lead 244,.overthe back contaetua-nd No. tarmathreof relay 2%, through resistance 243, andlover conductor 335 to the negative or lowerplate .ofcondenser 3I3. If a sufficient amountsof puresignal frequency is transmittedto. the discriminating network 3I2 the algebraic sum of the voltage on condenser M3 and theavoltage drop in resistance 52 :will'be a positive voltage suificient to produce a grid-to-cathode voltage at tube 31'! sufiicient V to permit plate current to'fiow to the extent that the remove-filter relay SIB and the receiver relay SIS will operate. The operating path for relays lit and 3? extends "from positive plate battery 352, through lampfifia potentiometer 3% for controlling the maximum plate current, through the winding of relay over the plate to cathode discharge. path within tube 3H, over conductor 355, over the upper pair of normal contacts of the D. C. test -jack, over conductor 35! and through thelower'winding. of relay 3 I 9 to ground. If, onsthe other hand, other frequencies than pure'signaltone frequencyare present in the composite signal transmitted tothe discriminating network {HZ-there will be voltages on both condensers 353 and Si l-which, as will be apparent, produce discharge..-currents which flow through resistance: 3 min the same direction. If the voltage drop in resistance 342 due to this combined discharge-current exceeds the voltage on condenser tit thegrid-to-cathode voltage of tubefil'l will be reduced below cut-01f. It should be apparent at this'point inthe description that the terms signal and guard channels are related to the .function-ingof' the receiver in that the signal channel alone (section 3I6) produces a voltage which tends'to operate the receiver by operating relays 3+8 and 359 Whereas the guard channel (section 3 I5) creates a voltage and in turn a discharge current which tends to guard against receiver operation.

The shunt path across resistance 34?, in the discharge p'athoi condensers M3 and 3M provides a delay in the build up of sufficient positive voltage on the gridtof :tubeti'ltoseiiect the operation of relays Md and 319 This delay is helpful in preventing receiver response due to shortpulscs of pure signal tone which might be present in voice currents. From a condition of no signal to a conditionof pure signal tone or other signal. containing pure signal tone and other vfrequencies varistor 3% and condenser 3% present practically wnoresistance at all to the discharge current and in effect short-circuit resistance 343. Theinet effect of this'transient short circuit-is :to produce a larger-discharge cur rent than'withcut the transient effect for a; relativelysmall voltage cn-eondenser- 3I3. This larger'dischargecurrenteffects a larger voltage drop in'resistances m to thereby delay-the build resistances 3'4 and-342.; through condenser 313, a

through resistance 349; through resistance 35% comprising partrofra potentiometerirnetwork from ground to negative battery 3 2'3,- rto ground,rthence through the lower "Winding of the crec'eive-r'relay 313, over conductor 35I, and :over the *upper two sets of normalcontacts of the DC. test jack to the cathode of tube 3H. If the input to the -re ing action of condenser tone signal in voice currents.

345 in delaying the response of tube 3!! to prevent receiver response to short pulses of pure tone signal which may appear in voice currents. After the transient period, however, the shunt- 346 will disappear. Therewill be little, if any, harmonics in a pure continuous tone signal but harmonics due to the limitin stage will still be present. The value of resistance 3343 becomes the minimum determinant of that portion of the discharge path.

Under the foregoing conditions it is apparent that the signal tone voltage pulses appear- "ingat the grid of tube 311 are foreshortened.

Another delay is provided by means of the effectively short-circuited upper winding of relay 319 upon operating energization of its lower winding, thereby making relay 3l9 slow operating which, as will be readily apparent from subsequent description, further delays the operation of the receiver with respect to its effect upon the signaling conductor H of the local trunk circuit. These two delays affecting the change of signaling conditions on the signaling conductor I 18 may be, as hereinbefore stated, referred to as electronic delay of the receiver. This electronic delay accomplishes two main purposes; namely, one to prevent receiver operation as a result of short pulses of pure tone signal frequency appearing in voice currents or otherwise and two, to foreshorten longer pulses of pure The slow operation of relay 3l9 delays achange in the condition of the signaling conductor I tothe local trunk circuit, beyond the delay caused by the receiver foreshortening of pulses and for some "short pulses will prevent such signaling al- I together.

This electronic delay will, of course, affect some normal signaling pulses but such pulses are of long enough duration to effect the'operation ofregeneration relay 208 which controls signal conductor H0 or are not materially affected by the-electronic delay due to thelowfacility. The special circuits and associated circuit elements concerning the regeneration relay 208 provide such lengthening. The relay 208 is operated in a circuit extending from ground over the armature and front contact of relay 3), lower normal contacts of the R test jack, over conductor 321, over the No. 1 armature and back" contact of relay 206, over conductor 2l3, and through resistance 2M and varistor 2l5 and the lower winding of relay 208 to negative'battery. When relay 208 releases, ground is applied to.

signaling conductor I10 over the right lower contact and armature of relay 208. At the same time condenser 246 is charged in a circuit extending from negative battery, through resistance 241 and varistor 248 and condenser 248 to ground over the two right front contacts and armature of relay 208. When relay 208 is again operated the armature will break from its right hand contacts This break permits the charge 7 ion condenser 245 to dischargethrough'alclr cuit traced from the upper plate of co'nt len's'er'.

246, through the upper winding of relay 208, and

- through resistance 249 and potentiometer 250 to the lower plate of condenser 246. This discharge current insures that the reoperation of relay 208 will be prolonged for a minimum duration to compensate for any of the aforementioned' electronic delay or foreshortening. The

adjustment of potentiometer 250 can insure a minimum length or duration of operation of relay 288 irrespective of receiver electronic delays.

The signal receiver is arranged, as hereinbefore described in detail, to prevent its own operation on short pulses of pure tone signal appearing in voice currents, to foreshorten all pulses of tone signal, to prevent transmission of longer pulses below a minimum duration of pure tone signal in voice currents to the local trunk circuit as a signal, to guarantee transmission of a minimum duration of intentional signal to the local trunk or further tool facilit and to transmit to the signaling conductor foreshortened signals caused by intentional tone signals of excessive duration.

Whenever the timing relay 253 is operated by the operation of the guard-removal relay 209 and the high level relay 202, condenser 348 is eifective- 1y inserted in parallel with condenser 3I3 of the signal channel discharge path to become charged in parallel therewith. The circuit is over the No. 1 front contacts of relay 253 to ground over the No. 6 front contacts of relay 202. Condenser 348 is of greater capacity than condenser M3 and consequently will maintain its charge longer. Under conditions where report signals are being transmitted over the toll line, as is explained subsequently, comprising on and off transmission of signal frequency impulses, the delaying action of condenser 348 will tend to keep tube 3!! operated to compensate for the foreshortening of the signal impulses caused by the guard channel 315 and by the circuit elements comprising resistances 343 and 344, varistor 345, and condenser 345. It will be appreciated from the subsequent description of report signal conditions that the receiving circuit at the first toll oilice is required to operate when it is arranged to be selective to incoming frequencies (due to the release of relay 209) and when it has the added effect of condenser 348; but, the receiving circuit is called upon to release when it is non-selective but still having the effect of condenser 348. If condenser 348 were not present the release and operate times of the receiver might differ appreciably in spite of the change in selectivity condition. The added effect of condenser 348 on the operate and release times of tube 3!! tends to equalize such time durations. In addition, the presence of the relatively large capacity 348 is insurance against the undesirable receiver release effect of possible discharge of condenser 3L3 on negative cycles of high power low frequency superimposed signal frequency, when the receiver is non-selective.

Idle condition of all circuits I (Continued) It is proper now to return to the description of the idle condition of the receiving circuits, having a detailed understanding of certain peculiar characteristics of the behavior of the receivers. It will be remembered that a continuous low levelpure-signal tone of limited amplitude was impressed upon the discriminating network 3 l 2 after having. been transmitted from the second toll office, over toll line TL2 or TL, through 515 itrans'former 265 iand :through the amplifying rand limitingwstage comprising tube-30 i, trans- 1-iformer3t3; varistorsfifi l. and the. low-pass filter -1comprisingretard coil ;-3fi5 rand 'condensers .306 and 331. :released, terminal 1: of "section -3l6 of the dis- =criminating networks 312 "is connected to the right -handend of resistance 339 overconductor 331, the No. 6 armature and back contact of relay 2:353, and over'conductor 336.

209 and 2&6 released section H is shunted by resistance-124:3 in a circuit extending .fromtermirnal -3:of -isection;3i5,-over lead 335;through resistance,243,:cverwthe No.";4 armature and back contact of relay 2%, and over lead 2 44 to terminal 4;of sectioneifi. Thus,iboth the guard channel "3-l5 and the signal channel 3+5 are effective, the guardzch-annel beinglimited'in its eiiectby the @resistance 243. Under the condition of continu- .:us.1ow level; pure signal tone thesignal-channel ":3 lfiwill chargeicondenser 3 l 3 but the, guard chaninelufiifi will not charge condenser 314 to :.any apracticalextent. As 3 previously explained condenser 3! 3 will discharge current from itstop plate-through resistances 342 and 343, through varistorfiefi, over conductor'M l, over the back :contact and Nomi armature of relay 266, through resistance 243, and over conductor 335 to the bottom. plate otcondenser filii. When low level ,puresignaltone is present the grid voltage of -tube ti'i will be such, as-a result of the voltage loncond'enser-sis and the voltages in the discharge path, that tube 3|! will pass sufficient plate current-to operate the remove-filter relay *3 iirinthe-plate circuit and the receiver relay 3 l 9 lint-the. cathode circuit, which'plate cathode cirucuithas-been traced previously. Relay 3 I 8, when operated .as shown-in Fig. 3, has no effect upon -the.block.ing network .329 because the sensitivity relay 2% is .quencypulsing is used, the front contact on .the armature 310. relay 256 would be permanently ecormectedato conductor such that when the relay-.318 operates it would insert filterBZfl. The sectionfifZ-l 'of network 32l-is in series between lead "'25! andthelgrid of tube 39% Section'322 is bridged-across the inward transmission channel composed'of conductors 25! and'252 by means ofaconductors 356' and'35'l over the'No. 3 armature rand :back conta'ct'of relay 2B6. Sections '32! and-322 are resonant at the incoming signal tone frequency and, therefore, prevent said signaling tone frombeing transmitted to the local Utrunk circuit through amplifier 300, transformer 358, overleadstti and sec, and through hybrid coil HCl. Relay 3H9, in-operating, operates the regeneration relay 2H8 in a circuit extending lfrom ground over the frontcontacts of relay'3 l9, over .the'lower normal contacts of the R test jack, 'overconductor .321, over the No. 1 armature and iback-lcontactof sensitivityyrelay 2'96, over con- :cluctonZlB, through resistance 2M andvaristor .2|.5, and through thelower winding ofzrelay 263 to negative battery. .As previously explained, relay 3i! 8,.in operating asshown in Fig. 2, removes ground from signalingdead. ill) connected to its armature. The lack of ground on conductor 11a 'is'an indication to the local trunk circuit that the 'receiving circuits are registering an onhook or idle condition of the remote offices call- *mal- 'contacts of'the R, test jack, over conductors With .thecguard removal relay 299,

With relays 7 released. However, if multifrefor obvious .,-.reasons.

@212 :and 2-H,- over thez-No. 5 armature--zand back contact" of relay @292, through varistor 210; and

through the winding of cut-ofirelay-ZM to .battery and ground. As previously explained, the

-:cperation of relay 252} effectively cuts and terminates the outward transmission channel, com- :prisingrti-p conductor and ring conductor-234,

to prevent transmission over toll line TLI' or TL of noise'or other. audibletones originating :in-the local trunk circuit.

fThe-receiveratthe second toll office functions the-same as the oneatthe first toll office described An idle conditionat both of- -Incthe :case of a two-wire .toll facility, however, lthesecond toll office must transmita diirerent, say 2000 cyclesper second, signaling frequency The respective receivers, under the influence-of continuous 'low level signaltone. incoming thereto, will, asabove described, operate.their'respectiveremove-filter relays (M8 and 5H3), receiverrelays (319 and5l9), :regenerationrelays (2G8 and 408) and cut-oil --relays ('2ll3'and 483). .The incoming'transmission channels at the respective offices (leads 25 I, 252 "from transformer 285 and leads 45!, 452-from -transformer=405) are blocked by fil er networks 320 and 520 -.to pi'eventtransmission of incoming signal tone tetherespective local trunk circuits through hybrid-coils HCl and H62. .-Furthermore, in the case where Fig. 4B may be specilocal signals.

"Transmissionand reception of a calling condition Assumenow that theplugAPl ofthecord-CDl at'the position OPJ in the .first toll ofiiceis/in- :serted into anansweringjaclr to answer a.call

incoming over a recording trunk oran .intertoll trunk ora trunk from, a community vdial office,

that the operator has .determinedthe destination .ofthe calland .thattheplug CPI of this cord CDI is inserted. into a jack-associated with anoutgoing trunk circuit over whichthe call maybe extended, and that the trunk ,OTi is associated With-the .jack J Linto which the plug-CPI .is inserted. ,The. insertion of plug .CP! into jack..,J I

.closes Ya circuit for. operating relay i fi l, .thereby erativelyv energizing the upper winding 1 of. sigductor. 110 from conductor. H39 therebysto prevent the operation of relay Hills ofltwo-way. trunk circuit TWTI when ground isv connectedtol sigznaling conductor :10, connects thewanswering .superrisorygrelayij'I G5 ..of the ,trunk..circuit OT 1 to conductor H0 andconnects groundto conductor 7 168 to: operate relay ice tor two-way trunk..cirjcuit TW'ITI. Relay H104 locks til-conductor. Hi8

anemia independently of relay [003 and further opens the connection between the winding of relay 15003 :and the signaling conductor 1 ll].

The operation of the outgoing :trunk signaling relay I40 connects negative :battery to conductor -I;64 through resistance I42 and over the righthand contact and operated swinger armatureoi relay I40. Negative battery on :lead 1-64 ;is applied through resistance 2L6 andover .lead 21:? "to the mid-point of resistance its to create a greater negative potential on the ileftehand .side :of varistors 2 I 9 and 1220 than created on the right-hand side of said Varistors by the potentir .ometer comprising ibattery 22.4 and resistors i2 2 and 223. This reversal .of polarity of ivoltage .across varistors '2 i9 and '2210 causes said varistors to assume .a relatively high internal resistance,

i. which maybe of the order-of 'azmillionohmsnr 7 so, which effectively prevents any of the output signal tone generated by signal generator 20s,;

nates .the marking relay .20! in .a circuit extending from ground, through the winding of relay 201, over conductor 2.1! and through resistance 216toresistancebattery on lead [64. "Relayzi l upon operating, completes an operating circuit for the ,high .level relay ,282 extending from ground,over the N0. '2 armature and front contacteof relay 2.01; over the No. ,2 armature and front contact offthe operated cut-01f relay 20,3, and tofbattery' and ground through the winding of relay 202; .Re'lay 1202 .operates in the latter circuit and locks independently of relay 20.3 over its No. 4 armature and .front contact .to ground over the No. 2 armature and ,front contact of relay 20L .Relay "282, in operating, opens the operating circuit of relay .263 which releases. Relay 203 isva slow releasing relay. In some types of second toll oflioes, such as panel, crossbar and special step-loy-step offiees, .the equipmentfor registering incoming direct current dial pulses or multifrequency dial pulse codes not immediately vavailable requiring that such-otfices transmit va 'delay dial signal to the re mote sending office to delay the transmission therefrom of such .dialing'information. Other types of second toll uofiicesusuch as ordinary step- Lby-step oifices, do not require such delay. At such called oflices, whichmustsend a ,delaydial signal to the originating ofiice, the relay corre- .,sponding to 203, namely relay 403, will not release if (such a 'delay dial signal is necessary. During the slow releasing time'of relay 203 its -.n ormal contacts of the Rntest jack, over =con- ,ductor 2l- 2, over thep'No. ,4 armatureand .fron't contactvof relay g20| i and to battery and ground :through theirwinding of relay 2.09;...Belay "2B9 operates in the latter circuit and flocks "indeipfindeintly ofzrelay 21 to groundoyer{itszNor ll nrmatureiand irontcontactmer thezNo. harmstureland front contactof relay 2 112,, over leads r211 and .212, and over the upper normal contacts of =the:Rtest;jack:to ground over theoperated contacts zof relayi3i9. Bielay 2119, in operating, com- -p1etes lacircuififcroperafing the timing r lay 5: extendin tram grou .d,:over the vI Q. 6 armature and front nontactiof relay .202, :over the No. 2 sarmatnre andif-ront :contact ;Qf relay 299 and to battery and aercun ztrhroue'h the winding of relay 32-53. Belay :3, upon operatin locks to g und, .over its :No- ,2 .zarmature and iront contact and .oyer atl e .NQ. i2 armature and ,back contact :of

sensitivity re ayifit.

The application cf ibat ery t0 l d L64 :at the local out oing trunk circuit QTI has, therefore, aresmted n the. nera icn .of the ma si ela 12 1, the higheve 1261345! 2,29 th en dw fimfi fi ilielay i209 and he timing relay (253, and the retardcdrl'cleasel f ith cut-iofirelay 203 to remoye the effective ,cut of the outward transmission path. Thehigih-levclirelay .202, upon operating, short-circuits 12118 series resistances i229 and 52,30 ,tmpermit subsequent transmission of .;signal fre- :quency zfro n i'slgnal generator #205 :over the ;outwaz d t ansmission path ;to :take ;place :at a level higher than the previouslyadescribed idlecondition low level signal. Thesevshortecircuitin gpaths :may be traced vfrom .the zleft .side .or resistance 229 to the right side thereof lover :the No. :2 armature and front icontact of :relay 232 and ,from the .-lef.t to the :right :side .of resistance 2311 ;over the No, 1 armature.andsironticontactloi zrelay 2H2. .fIiheloperation-of the guard-removal relay 2.99, as has :been described heretofore in detail,.:changes .theicharacteristicsiof the local re;- ceiver to :make it znon=selective to incoming tre iquencies. This as accomplished, as vwill be :re- .melnbered, :by short-.circuiting the terminals .3 and cl ofi'the guard channel 315 :over leads. 335 and 324-4 and over zthe :No. ;5 armature sand l'fron't contact ofirelay 209 and by removing the short- ..circuitacross resistance 245 at the No. 6 armaiture and back :contactzof relay-z209 to therebyin- ,sert resistance :-2 4.5. iniseries ,withleads5336 and 33'! connecting terminal I of ztheisignal-channel 331:6 to the right :side of resistance I339. This-change ii-n qselectivity of the :receiver will not alter the operation of relays 31.8 :and 32l'9 i-zsince w level signal tone :is still incoming and will charge condenser} I 3 ;as:before. vThe guard-removal 'relay i209, being iqui'ck operating, :prevented the sensitivity relayizflfi from operating over the No. 1 iarmature and front contact of relay 209 rebut 1.permittedztheitiming :relay :2153 to operate follow-- :ing :the operation iof :the ihigh level relay 202, whereupon irelay :3 looked under the control of the releasedsensitivity relay 206. Theiopera- -'tion of the timing irelay -253 places additional short circuits across' eseries resistances 229 and v123i! of the signal tone:transnm'ttin eircuitiover i sfNo. j.3 and, No. 4 armatureszand front contacts puer obviouszcircuits. :Relay253,aupon operating, also applies positive zbatter ithrough lamp -25?! to the :main anode :255 .of the gas discharge timing itube 25.6 aoyer :the :No. .5 armature and iron-tcontact 0f;relays25 3. Condenserz25fLinithe triggering circuit of ithe :starting :anode 258 oi ztubea256, is prevented from charging up to the triggering potential due to I the ground :app'lied to :the right-hand end of resistance 259 over lead 2260 and over the No. 3 armature and front con :tact;of.relayi20'9. v 7 ilheidiscontinuance' of the ftransmi ssion i signal :toneitrom the cfirst' tdll oflice is recognized at varistor 56 I.

ing seized on an outgoing call.

the receiverof the second toll ofilce as a calling signal from the first office. The removal of signal tone from toll line TLI and from conductors 530, 440, 442 and MI, comprising in conjunction with transformer 405 the input circuit for tube 50I of the receiver at the second toll oflice, removes the grid input voltage to tube 511 resulting in a reduction or complete cessation of plate current fiow in tube 5I1 to thereby release the remove-filter relay 5 I 6 and to release the receiver relay 5I9, which, although slow operating, is fast releasing due to the unidirectional character of its contacts the operating circuit for the regeneration, relay 408, which thereupon releases. The release of relay 408 applies ground over its lower left contact and swinger armature to signaling -lead610 to the local trunk circuit. The release of relay 5I9 also opens the operating path of the cut-off relay 403 which proceeds to release slowly and will completely release because no delay dial signal is received from the local trunk circuit before it can release. The release of the remove-filter relay 5I8 has no effect at this time,

except in the case of multifrequency dialing, as above described, whereupon the release of relay 5IB removes the filter 520.

The release of relay 408, in addition to applying ground to signaling conductor 610 to initiate the trunk circuit operation to be described directly, permits condenser 446 to acquire a charge in the circuit from ground and negative battery through resistance 441 and varistor 448, through condenser 446 and over both left-hand contacts and swinger armature of relay 408 back to ground. Aspreviously explained, the charge on condenser 446 is utilized in making relay 408 slow releasing once it has been reoperated to insure a full length signal (comprising an open-circuited lead 610) to the trunk circuit irrespective of the previously described foreshortening of signal tone pulses and electronic delays of the receiver.

Ground on conductor 610 causes the operation of relay 6003 of the two-way trunk circuit TWT2 in a circuit extending from ground on lead 610, over the back contacts of relay 6I0 of trunk circuit T2, over conductor 669, over the back con- 'tacts of relay 6004 of trunk circuit TWTZ, over the back contacts of relay 6001, and to battery and ground throughthe winding of relay 6003.

Relay 6003, in operating, connects ground over its No. 4 armature and front contact to conduc tor 668 to guard the trunk circuit 0T2 from be- Relay 6003, in operating, also completes a circuit for operating relay 6005 of the trunk circuit TWTZ and for the line relay 6051 of the trunk route selector TS2. The latter circuit may be traced from ground on lead 610, over the back contacts of relay N0 of trunk circuit OT2,over lead 669, over the back contacts of relay 6004 of trunk circuit TWT2, through the winding of relay 6005, over the No. 1 armature and front contact of relay 6003, over lead 661, through resistances 62I and 622 in simplex, over the back contacts of relay 620, through the windings of the retard coil 660, over conductors 66I and 662, over the Nos. 2 and 3 armatures and back contacts of relay 630, over conductors 600i and 6002, over the Nos. 2 and 3 armatures and front contacts of relay 6003, over the Nos. 1 and 2 armatures and back contacts of relay 6060 of the selector T82, through resistances 606I and 6062 in simplex, through resistance .6063, over the No. 5 armature and back contact of relay 6060, to battery and ground through the Relay 5I9, in releasing, opens at winding of relay 6051. Relays 6005 and 6051 both operate over the latter circuit. Relay 6005, upon operating, closes a circuit from ground, over its No. 2 armature and front contact and through resistance 6006 to the winding of relay 6003 to hold same operated. Relay 6005, in operating, also completes an obvious circuit for operating the slow-to-release relay 6001. Relay 6001, upon operating, closes an additional circuit from ground over its No. 1 armature and front contact for holding relay 6003 operated, opens at its No. 2 armature and back contact the operating circuit for relay 6003 and connects ground over its No. 3 armature and front contact to conductor 6053 of selector TSZ to hold the selector T82 and any additional selector through which the connection is extended in the second toll ofiice until the connection is released at the first toll oifice.

Dialing routing digit If the dialing is to be accomplished on a multifrequency basis the circuits at both oflices remain ineffective so long as the frequencies used in the multifrequency dial signaling do not comprise the supervisory signal tone frequencies used by the two OfilCBS.

It being assumed that no delay in dialing is necessary (the effect of a delay or stop dial signal will be described hereinafter), if the dialing is to be accomplished on a dial pulse basis the operator at the first toll ofiice operates the dialing key (not shown) to place the upper winding of relay I40 under the control of the impulse contacts of the dial at position OPI over conductor. E02, the No. 4 armature and back contact of relay I30, over conductor I06 to ground through the upper winding of relay I40. Each release and reoperation of relay I40 in response to the dialing of the first routing digit of the called number alternately places ground and battery on conductor I64. Ground on leads I64 and 2I1, as has been explained, reverses the polarity of voltage across varistors 2I9 and 220 to again permit transmission of signal tone over toll line TLI or TL, this time at a high level since relay is operated thereby short-circuiting the series resistances 229 and 230. Ground on leads I64 and 2H also permits relay 20I to release. The release of relay 20I completes the operating circuit for relay 203 from ground, over the No. 2 armature and back contact of relay 20I, over the No. 3 armature and front contact of relay 202, through varistor 26I, to battery and ground through the winding of relay 203. When battery is again placed upon conductors I64 and 2H, relay 20I reoperates. Relays 203 and 202 remain operated during impulses of a digit due to their slow releasing characteristics and relay 20I releases during the impulses and reoperates at the ends thereof. At the end of the digit, relays 20I and 202 remain operated and relay 203 will slowly release. Each time that leads I64 and 2I1 are changed from battery-to-ground-to-battery a single pulse of signal tone is transmitted to the second toll office over toll line TLI or TL.

At the receiver of the second toll office the arrival of each tone pulse causes the operation of the receiver relay 5I9 and the remove-filter relay 5I8 as hereinbefore described. The relay 5I8, in operating accomplishes no useful function at this time. Relay 5I9, in operating, causes the operation of the regeneration relay 408 to thereby remove ground from lead 610 .to the local trunk circuit. When each incoming tone pulse ceases,

the relays 5I8,.5I9 and 408 release to thereby in response of thereceiver'at the second toll office 'to'the dialed tone pulses, relays 6005 an'dfi051 release. Also each time lead 510 is grounded re- -lays 6005 and 6051 reoperate. -Relay -t'flfllg-whose operating circuit is undercontrol of :relay 005,5is slow releasing and is unaffected by them'omentary release of relay 15005 3 1 response to each pulse of each digit. Each release of relay 605.1

causes the operation of the vertical stepping mag- 'net (not shown) of selector -TS2 whereby the brushes 6011, 6012, 6013and-6014 are s tepped up to the level correspondingto thedigit-dialed. At the'end of this train of impulses, thebrushesare advanced step by step in the selected level until onset of "terminals connected'to an idle trunk or succeeding selector are encountered; whereupon relay'5060 is operated to extend the connection to "the selected switch or trunk, all "in the usual andwell-knownmanner.

Each succeeding train of dial impulses created by operation of the calling operator s difal is repeated by-relay I40 Which-alters the transmitting character of varistors --2-l9 and 22-0 to 'transm'it' a corresponding train of impulses of -voice fre- 'quency signaling current over tollline or I-"L to operate relay 408 and the operation of relay '408 effects the transmission of corresponding trains of impulses, through brushes' fifl'll and 6012 and the selected terminals of selector "T82, to operate succeeding switches through which the connection is extended or to operate digit reg- *isters. When dialing is completed, the dial key (not shown) is restored to normal -and' relay l im operated in the manner describedfin-the aforementioned King-Miller and Walsh patents-"to complete the talking connection between cor'd -CDI and hybridcoil H01.

,Tmnsmisszbn and receptionrqf a fdeZaydiaZf -Qr sltop dial signal Y The variouslevels .onrselector Ts-lrnayzrepre- :sent 1:011 routes to various :types offterminating or tandem equipmentsuch aspanel cross bar,:or slink-type .step-by-step ofilces which contain equipment designed to register incoming. mulse V signaL'whichequipment isnot immediatelynvaileor delayed until such equipment isava'ilable for operated as soon as "brush f60-l'4 Of selector IlSZ snakes contact with terminal 6015 {randy-as soon as 1 connector relay 6060 operatespas described -"i n 22 the aforementioned Kin'g-Mill'er patent. Relay 6034, in'opera'ting, completes the operating cincuitifor relay from ground, over the front contacts of relay 034, terminal -15 and brush -6014-of selector TS2,:over the No. 4 armature and front :contactof connector relay 5060, to battery and ,ground through the winding of relay 60H. Relay- 6M l,-=up0n operating, places resistance battery on lead 664 from battery through resistance 642, over the front contacts of relay 60M, and over the swinger armatures and right-hand contacts o'f relays -60 I 0 and'640. Battery on lead 664 causes varis'tors l-lfi and #420 to assume high imiped'ances to'eifectively-cause cessation of lowlevel transmission of tone signal to :the first toll o'fiice. Battery on lead 664 also scauses operation of relay 401 :over an obvious circuit. If the cut-off relay 4'03 "has released, the operation of relay dll l will reoperate relay 403 in a circuit extending from ground, over the No. '2 armature and front contact io'f relay 40L over the No.3 armatureand back contactof the high-level relay 462,-through varistor 461, and to battery and ground through :the windingof relay 403. Relay 403, inoperat ing, completes the operating circuit for relay 402 from ground, over the No.'2'armature and front contact of relay 4M, over conductor 482, over the :No. 2 armature and front contact of relay .403, to battery and ground through the winding of relayf402. Relay 402, upon operating, opens ati-ts N0. 3 armature and back-contact, the operating circuit for relay 403 which slowly begins to release. If the cut-off relay 403 had not released atzthe time the relay 40! was operated, relay402 will "be operated as above followed by the slow release of relay 403. In either case relay-403 will eventually release to removetheefiective cut and termination of the outward transmission channel to toll line TL2 or TL. Since the "receiver relay .519 is released, the operation'of relay 401 will not operate the guard-removal relay 409, and, therefore, -a circuit is completed for operating :the sensitivity relay 406 from ground, over the N0; '6 armature and front contact of the operated relay 402, over the No. 1 armature and back contact-of the timing relay 453, over the No. 1 armature and-back contact of relay 309, over the No.1 armature and front contact of marking relay 40 I, andto battery and ground through the winding of sensitivity relay dflfi. In addition, in the case "of two-Wire toll'line operation as shownin Fig. 4B, the operation-of relays 402 and 40! provides an operating path for the filter relay '300l extending from the previously traced ground, over the No. 1 armature and front contact of relay 401, over conductor 463;to battery and ground through the winding of filter relay 300 I. The operation of "relay 300I removes from the inward transmission 'channela network 3004 tuned to the outgoing tone .rering relay 464 over its No. 2 armature andfront contactto ground. Relay-464, in operating, provides over its Np. 2 armatur :andfront contact anadditional ground for holding-=relay 402 operated "over the No. 4 armature and frontcontact ofn'elay lM. Relay 464 partially completes at its 23 No'; 3' armature and front contact a circuit from ground, over the No. 3 armature and back contact of relay 409 to the No. 4 armature of relay 40I and from the lowermost back contact of relay 40I, over the No. 3 armature and front contact of relay 464, over conductor 465, over the lowermost normal contacts of the D. 0. test jack, and over conductor 562 to the left side of the winding of relay 40I, whereby when relay 40! releases, as a result of battery being removed from conductor 664, the latter circuit will be completed over the lowermost back contacts of relay 40I and said relay cannot be reoperated until slow-releasing relay 464 releases to remove the ground shunt across the winding of relay 40I. This delay will insure a minimum length of transmitted tone from the second toll ofiice to the first when battery is removed from conductor 664. Furthermore, when relay 464, is operated, it completes an obvious operating circuit for the out-filter relay 466 to ground over the No. l armature and front contact of relay 464. The operation of relay 466 removes from the outward transmission channel the blocking network 467 which is tuned to the signal tone frequency transmitted normally from the second toll office.

The discontinuance of the signal tone transmission to the first toll office results in the release of the remove-filter relay Hi6 and of the receiver relay 3I9. The release of relay 3I9 results in the release of the regeneration relay 208. Relay 208, in releasing, connects ground to conductor I10 which is interpreted by the local trunk circuit as a stop dial signal. Ground on conductor I19 operates the supervisory relay I05 of trunk circuit OTI over the No. l armature and front contact of relay H6. Relay I05, upon operating, opens at its No. 2 armature and back contact the circuit including the low resistance winding of relay104 and the calling supervisory lamp CS. Negative battery is connected to conductor I06 over the No. l armature and front contact of relay I05 but this has no effect upon relay I40 since negative battery is already connected thereto from the operators dial normal. Lamp CS is extinguished to inform the operator at position OP! that further dialing should be forestalled. The release of the receiver relay 3I9 releases relay 209. The release of relay 209 removes at its No. 3 armature and front contact the ground on lead 269 thereby allowing condenser 251 of the timing circuit to begin to acquire a charge.

If the stop dial signal is of sufilcient'duration condenser 25? will become charged to the striking potential of tube 256 whereupon tube 256 fires and produces a discharge current which flows through the lower winding of the differene tially wound timing relay 253 to neutralize the efiect of the upper energized winding. Relay 253 will thereupon release to complete an operating circuit for the sensitivity relay 206 extending from ground, over the No. 6 armature and front contact of relay 202, over the No. 1 armatures and back contacts of relays 253 and 209, over the No. 1 armature and front contact of relay 20I, to battery and ground through the winding of relay 286. Relay 266, upon operating, in addition to altering the sensitivity of the receiver, completes an obvious circuit for operating the rering relay 264 at the No. 2 armature and front contact of relay 206 and opens at its No. 1 armature and back contact the operating circuit for relay 208. Rering relay 264, in opers g. rovides r. its 2 ar r an additional holding ground for the high-level relay 202, completes at its No. 3 armature and front contact an obvious circuit for operating relay 266, and at its No. l armature and front contact partially completes the shunting ground for the right-hand side of relay 20I to insure a minimum release time for relay 20I when battery is removed from leads I64 and 2H as has been explained in connection with similar relays of the second toll oifice.

If the stop dial signal is of short duration the timing relay 253 will not release because condenser 251 will not have had sufiiclent time to acquire a voltage which will initiate a discharge in tube 256. Therefore, neither relay 206 nor relay 264 nor relay 266 will operate before relay 209 reoperates, as will be explained, to recondition the control relays 209, 206, 253, 264 and 266 as they were before the stop dial signal was received.

Transmission and reception of a start dial signal When the terminating equipment is ready to receive pulses, relay 6034 of the selector T82 is released thereby releasing relay 66H which replaces the battery on lead 664 by ground. Ground on lead 664 reverses the polarity of voltage across varistors M9 and 420 to thereby allow their internal resistances to decrease to effectively cause the immediate transmission of tone signal over toll line TM or TL. Such signal will be transmitted at a high level because relay 402 is operated, thereby short-circuiting the series resistances 429 and 430. Ground on conductor 664 shunts the winding of the marking relay 40I which thereupon releases. The release of relay 40I, completes an operating circuit for the cut-off relay 403 from ground, over the No. 2 armature and back contact of relay 49H, over the No. 3 armature and front contact of relay 492, to battery and ground through varistor 46I and the winding of relay 493, which relay thereupon operates to cut and terminate the outward transmission path. The release of relay 46I releases the filter relay 300I, if a two-wire toll line is in use, to insert network 3004 into the inward transmission path at the second toll ofiice to greatly reduce the efiect upon the receiver at the second toll oflice of transhybrid or echo currents of the signal frequency transmitted from the second toll oilice. Along with the release of the filter relay 300I, the release of relay 49I causes the release of the sensitivity relay 466. Relay 496, upon releasing, returns the receiver to its normal sensitivity and normal signal-to-guard efficiency, and at its No. 1 armature and back contact partially recloses the operating circuit for relay 498. The release of relay 406 opens the operating circuit for relay 464 which releases slowly due to the short-circuiting efiect of varistor'468 and potentiometer 469. Relays 402 and 403 will remain operated until relay 464 releases. During the releasing time of relay 464 when relay 4&2 remains operated, high level tone will be transmitted. When relay 464 releases, relay 466 will release and relay 402 also releases to thereby reinsert series resistances 429 and 430 into the output circuit from tone generator 400 to thereby decrease the level of transmission of tone signal. Relay 403 slowly releases following the release of relay 402, as has been explained, to remove the cut and termination of the outward transmission channel.

At the first toll office, the reception of the high level tone si na1,causes the receiver to operate amine the remove-fllterrelay'3458: and the: receiver relay 31.9.. The. relay 3I8,, in. operating. rapidly reins'erts. the. blocking, network 320. into. the. inward transmission path. if" the relay 206. is operated, as will be. remembered. was. the case where the stop dial signal was of suflicient' duration. to cause the release. of. the timing. relay 253. Thus, only a small spurt. of incoming signal tone. is permitted; to be transmitted to and through hy- Brid coil 1-10]. It the sensitivity relay 20.6 was unoperated', as was the. case when the. stop dial."

signal wasof insufficient duration to. cause the release of the timing rel'ay"253,1the operation of the relay: l at this. ti'mehas no effect because. the filiter'32'0-is already inserted" in the incoming channel'.. Asl willlbe remembered,v howevenif multifrequency signaling. employed, thev operation of relay.- 318 would. insert filter 320. irrespective of the status of. relay 2061. If the relay 206 were unoperated'g, the operation of the receiver relay 3.1.92 operates the regeneration relay 20B. in.a prevlously traced. circuit over the. front contact. of relay 319' and over. the. back contact. and. No- 1 armature. of relay 205. The operation of relay 20.8. removes ground from. lead I to the. local trunk. circuit as a start dial'signal.. The operatlbnofrelay 3'l9 'al'so operates the guard-removal relay 20.9 in a previously. traced; circuit over the front contact of relay 3L9 and over the front contact and No. 4 armature of relay 201.. The relay 209', in operating. grounds lead 260 to. prevent. the. timing. tube. 256 from triggering and causesthe receiver to become non-selective to incoming frequencies. If. relay 206 were-operated and" relays 209,, 208 andl253' were released as a result ofa sufficiently long stop dial pulse to trigger tube 256,, then the operation of receiver relay 3.1.9 causes theoperation of relay 209 and relay 253. The operationv of relay 209 causes the release of relay 206,, which is slow releasing dueto' the short-ciicuiting effect of 'vari'stor 210 and potentiometer. 21! over the No.. 1 armature andifront contact of relay l'. The release of'rela'y2064 completes at its No. I armature andlo'ack contact the operating circuit for. relay 200 which operates to remove. ground fromlead 1'10 to the local trunk circuit. The release of'relay 206 releases slow-releasing relay 264 which in turn releases relay. 266; As a result of these relay operations, as will be remembered" from previous description, the circuit at the first toll office is ready to transmit high level tonesignal, its receiver is arranged for low sensitivity butis not selective to incoming. frequenciesv and" all. block'- lngnetworks are effective; Removal. of ground from conductor I10 releases; relay I 05 thereby completing at" the No. 2 armature and back contact of" relay I055 a circuit through thelowresl'stance lower windingof relay E04 and the calling; supervisoryl'amp CSLto battery and. ground to cause. lamp CS to light to thereby provide the operatoriwith a startdialf"signal;

Dialing-succeeding digits- As: previously out-lined,. if; succeeding switch operations"- are necessarmeach operation of the dial attheoperators positionOPl creates a. train or: impulses which. are. repeated? by relay I40 which; alters; the transmitting character of varistors 219.: and 220 to transmit: a correspond: ing train: of impulses-.ofvoicefrequency. signalinggcurrent over thetolllineJILt' or TL to control the: relay 400;. and; the; operation of relay- 408 im response. toathese; impulses: of voice frequency cnrrentefiects the transmission: of. corresponding 26-" trains of. impulses, through brushes 00' and 6012 and over the: selected: terminals of selector TSZ, to operate succeeding switches through which the connectiorr'is extended or to operate dig-it registers. WhendiaIing is completed, the dial key (not shown)" is restored to normal and relay 1-30 is operated: in the manner described inthe aforementioned" King-Miller' and Walsh patents to complete the talking connection between' cord' CDI: and hybrid coil H'Cl'.

Called subscriber answers- It will be assumed that the call in question is one which isrouted; through selector TSZ and over a level thereof which terminates in a selected idle incoming trunk circuit to a toll operators' position in the second toll office and that the selected trunk circuit is the trunk circuit 1T2 terminatingat' jack J8. In this" case relay 6034 will not be operated, as inthe case when a stop dial" signal was necessary. However, the operation of connector relay 60.50 of selector TSZ closes a circuit for operating. line relay 6055 of the trunk circuit 1T2: This circuit is traced from ground on the lower'left contact of relay 408, over the swinger armature of relay 408, over conductor 610, over the No. 2 arma-. ture" and back contact of relay 010 of the outgoing trunk circuit 0T2", over conductor 669, over the back contact of relay 6004, through the Winding of rel'ay60'05i over theNo. 1 armature and front contact of relay 6003, over. cond'uctor 601, through resistances 62! and 622'in simplex, over the back contacts of relay 520, through the windings of retard coil 660, over conductors 66! and 662, over: the Nos; 2' and 3 armaturesand back contacts of relay 630, over conductors 600 i and 6002, over the Nos; 2 and 3 armatures and front contacts of relay 5003, over the Nos. I and 2 armatures and front contacts of relay 6060, through". brushes 601i and 6012 of'selectorTS2, over" conductors 602i and 6022 of trunk circuit 1T2 through the windings of retard'gcoil 6000,through resistors 0M5, 6M6 and 0011, over the back contacts of relay 60m in simplex, 'to battery and ground through the winding of relay' 6055. Relay 6055 operates in series with. relay {i005 which holds relay 600'! operated to maintain ground. on conductor $053 Re1ay'6055,, in operating, closes an obvious circuit for lighting lamp L8. When the plug of a cord CD2. at a position. OP}! is inserted into iack J8 to answer the call,, the lamp L8, is ex.- tinguished'; and a talking, connection is com.- pleted through condensers 605.0 andfi05i... Relay 6034 is operated in the. manner describedin the. af'orementionedpaknt.to J. E. Walshand closes a. circuit. from. ground, over. conductor 6e44,. over terminal. 6.015. and. brush.'601.4- of. selector TS2, over the. No... 4. armature and frontcontact. of relay 606.0,. to. battery and ground through the. winding. of. relay 50.. of the two-way trunk circuit- TWT2.. The operation. of relay till-t reiplaces. ground on. conductor 554' by battery through. a: resistance 642*. Battery on. conductor B10 efiectivelycausescessation of low'leveL signal. tone transmission over toll line'TLZ or TL. Battery on conductor 61 0 operates relay 40L which operates the cut-off. relay 403 which in turn permits the! operation: of relay 402 which in. turnpermits; relay 403: to slowly release,. as has-been fully described; hereinbefore in connecttionwith the efiect of. a 'stop.-dial! signal.- Also, as has been described the operation of relays 401 and. 402 completes the operating circuits for 

